Investments and Cash - Distribution of Fixed-Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Estimated Fair Value
Amortized Cost	$ 10,814	$ 10,564
Estimated fair value	11,301	10,823
Fixed Maturities [Member]
Amortized Cost
Due within one year	254
Due after one year through five years	1,574
Due after five years through 10 years	2,368
Due after 10 years	4,599
Estimated Fair Value
Due within one year	256
Due after one year through five years	1,604
Due after five years through 10 years	2,443
Due after 10 years	4,961
Amortized Cost	10,187	9,974
Estimated fair value	10,674	10,233
Fixed Maturities [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Estimated Fair Value
Amortized Cost	852	998
Estimated fair value	861	987
Fixed Maturities [Member] | Commercial Mortgage Backed Securities (CMBS) [Member]
Estimated Fair Value
Amortized Cost	540	575
Estimated fair value	$ 549	$ 583